The present invention relates generally to photovoltaic systems with an inverter for transforming an electric DC voltage into at least one AC voltage. The invention resides in particular in an apparatus for providing an input DC voltage for a photovoltaic inverter, a photovoltaic system including such an apparatus and a method for operating the photovoltaic system.
Photovoltaic (PV) systems for generating electric energy include a PV inverter which converts a DC voltage generated by a PV generator into an AC voltage which them can be fed for example into an energy supply system. Suitable inverters, in particular inverters with a bridge circuit consisting of semiconductor switches and associated control methods are generally known and described for example in DE 10 2005 024 465 B4.
Known inverter circuits with semiconductor bridges have been found in practice to be reliable in the feeding energy generated by means of photovoltaic systems into an energy supply system. However, the DC voltages present in an intermediate circuit at the DC voltage input of the inverter need to have at least the value of the peak amplitude of the energy supply system. Otherwise, the voltage is insufficient for generating the desired AC current or respectively the desired AC voltage. Consequently, the photovoltaic generators used need to provide a sufficiently high intermediate circuit voltage.
Existing installation regulations often permit only limited maximum voltages. In the U.S., for example, only PV systems are permitted to be installed whose voltages, as measured to ground, does not exceed 600V. This must be ensured in particular also during operating disturbances, for example, in case of a short circuit. Since operating disturbances during feeding of elective power into the power supply system can never be excluded, PV generators for the USA are generally so dimensioned that their idle voltage does not exceed 600V DC.
Even though different AC current or three phase current supply systems are used in the U.S. a normal three phase power supply system has a system voltage with an effective value of 3 AC 480 V. This results in the need for an intermediate circuit voltage at the input to the inverter of at least 800V DC for feeding the power unto the net. This intermediate circuit voltage must be generated normally by expensive and inefficient DC/DC voltage increasing devices in particular because of the regulations limiting the maximum voltages. The use of voltage increasing devices in an inverter circuit however results necessarily to a lower efficiency and an increase in the complexity of the PV system.
Another possibility to generate a higher AC voltage resides in transforming the AC at the exit of the inverter by means of a transformer to an AC voltage with higher amplitudes. Such a transformer however also causes relatively high losses and substantially reduces the efficiency of the photo voltaic system. There is in the US the tendency to use photovoltaic systems with inverters which do not require any transformers.
US 2009/0032082 A1 proposes to couple two PV generators in series when needed to provide with the maximum admissible generator voltage a higher intermediate circuit voltage. The PV system disclosed includes a first PV generator with a positive and a first neutral connection, a second PV generator with a negative and a second neutral connection, an inverter which is connected to the positive connection of the first PV generator and the negative connection of the second PV-generator and a remotely controlled circuit arrangement. The circuit arrangement includes a main switch which is closed to connect the first neutral connection of the first PV generator to the second neutral connection of the second PV generator when the two PV-generators supply power to the inverter, and which is opened for separating the two neutral connections from each other, when the two PV generators do not supply power to the inverter. The PV generators are each designed for a maximum idle voltage of 600 V DC, which, during operation provides for a maximum intermediate circuit voltage of 1200V DC without the use of a voltage increasing device. However, when at the neutral connections of the two PV-generators a potential corresponding to ground potential occurs the limitation of the system voltage to maximally +/−600 V DC is observed.
When the PV-generators do not supply any power to the inverter, for example, at night or a fault they are uncoupled by opening the main switch. For this case, the circuit arrangement includes further auxiliary switches which are provided to connect the neutral connections of the PV-generators in each case to a ground connection at the inverter. In this way, the two PV generators are isolated from each other and are grounded.
This configuration however is very complex in its setup and, additionally requires expenses for circuitry and a control of the operation. It requires three DC switches which used to be controlled to establish the two modes of operation with coupled and uncoupled generators. In addition conduits for the grounding of the neutral connections of the two generators via the auxiliary switches are required.
In addition also an isolation measurement with uncoupled PV-generators is more expensive. It is important to monitor the isolation at least before the connection of the inverter to the power supply and also driving operation. In this connection, it is generally known to input predetermined signals at the DC voltage side of the inverter and to evaluate the voltages or currents obtained as a reaction to those signals in order to determine anomalies. The configuration known from US 2009/0032082 two separate circuits which need to be maintained separately are present when, out of operation, the main switch is opened and the auxiliary switched are closed. Consequently for each circuit separate means must be provided for measuring the isolation and the isolation measurements must be performed independently of one another.
Based hereon it is the object of the present invention to eliminate the shortcomings of the state of the art or at least to reduce them and to develop a cost-effective and reliable arrangement for providing an input DC voltage for a photovoltaic inverter with which a reliable maximum generator voltage (of for example 600 V DC) against ground can be maintained in each operating state while still maintaining the advantages of higher intermediate circuit voltages above this limit value with regard to an efficient feeding of the power into the power supply. This is obtained with a simple design and low expenditures. Furthermore the arrangement should also facilitate a low-expenditure isolation measurement.
It is a further object of the present invention to provide a photovoltaic system with such a voltage providing arrangement and an associated operating method.